The present invention relates generally to a circuit breaker. More specifically the present invention relates to an arc fault circuit breaker, wherein voltage is sensed across a bimetallic element and processed by current sensing components to detect the existence of an arc fault.
Arc fault circuit breakers typically comprise a pair of separable contacts that open (trip) upon sensing an arcing current from line to ground, and/or from line to neutral. Arc fault circuit breakers typically use a differential transformer to measure arcing from line to ground. Detecting arcing from line to neutral is accomplished by detecting rapid changes in load current by measuring voltage drop across a relatively constant resistance, usually a bimetallic element (bimetal). Additionally, during over current conditions (i.e., above rated current) the bimetal heats up and flexes a predetermined distance to engage a primary tripping mechanism and trip the circuit breaker.
Components of arc fault circuit breakers are generally assembled into separate compartments as defined by their function. More specifically, mechanical components (e.g., load current carrying and switching components) of each pole are assembled into mechanical compartments, while the current sensing components are assembled into an electronics compartment. In order to connect the compartments, the load current of each pole must be routed from the mechanical compartments into the electronics compartment, through appropriate current sensing devices, and back into the mechanical compartments. Additionally, conductors or sensing lines (e.g., wires connected to the bimetal), must also be routed from the mechanical compartment into the electronics compartment.
The bimetal has a dual function. First, it engages the circuit breaker's primary tripping mechanism to trip the circuit breaker during over current conditions (e.g., above its rated current of 10, 15 or 20 amps). Second, it also detects multiple, instantaneous, high-current arcing (e.g., 70 to 500 amps or more) from line to neutral.
For the first function, the bimetal is constructed of a pair of dissimilar metallic strips having different coefficients of expansion. When the bimetal conducts current, the dissimilar metallic strips heat up and expand at different rates, causing the bimetal to flex proportionally to the current conducting through it. The bimetal is calibrated to flex a predetermined distance during over current conditions to engage and activate the tripping mechanism. This, however, requires a relatively large amount of space within an already cramped mechanical compartment to accommodate the free movement of the bimetal. This problem is exacerbated by having too many connections attached to the bimetal which must also be allowed to move freely as the bimetal flexes. Additionally, making too many connections to the bimetal during assembly may bend the bimetal enough to throw it out of calibration. Therefore it is desirable to keep to a minimum, the number of connections to the bimetal.
The second function utilizes the relatively constant resistance of the bimetal. The voltage drop across the bimetal, is sensed by sensing lines and processed by circuitry (e.g., a printed circuit board) located in the electronics compartment to detect the arcing. When voltage drops indicative of arcing are detected, the circuitry generates a trip signal to activate the tripping mechanism and trip the circuit breaker. However, voltage drops indicating an arc fault are small and rapid, and can be imitated by electromagnetic interference (EMI) in the sensing lines. If the sensing lines are not properly protected, EMI may cause the sensing circuitry to trip the circuit breaker without the occurrence of arcing (false trip).
In order to reduce the effects of EMI on prior art circuit breakers a pair of sensing lines (e.g., wires) are first connected to the printed circuit board at assembly. The lines are then twisted together to offset the effects of EMI before they are routed through appropriate openings into the mechanical compartment, where they are connected across the bimetal. However, the twisting process is labor intensive and problematically adds to the cost of assembly.
In an alternative prior art embodiment, a pair of shielded wires (e.g., coaxial cables) are used as sensing lines to reduce the effects of EMI. However, shielded wires are expensive and still require connecting two wires across the bimetal in the cramped mechanical compartment, which can result in disturbing the sensitive calibration of the bimetal.